1. Field of the Invention
The present invention relates to magnetic recording medium, and more particularly, to magnetic recording medium having excellent electromagnetic conversion characteristics and excellent running durability.
2. Disclosure of the Related Art
A typical magnetic recording medium comprises of a magnetic layer on one side of a non-magnetic support and a back coat layer on the other side of the support. In helical recording, which has been the major recording scheme in conventional magnetic recording systems, the running speed of the tape itself is not so fast in spite of the relatively high running speed of the tape relative to the recording head. Thus, the running or recording performance of the tape is not significantly affected in helical recording if the back coat layer has a proper strength.
In order to control the coating strength of a back coat layer, inorganic powder such as a-alumina (i.e., an abrasive) is added to the back coat layer. For example, Japanese Patent Publication No. Hei 5-63842 (1993) discloses a back coat layer comprising an electron beam-curable resin binder composition that contains at least one of an electro-conductive filler powder and an inorganic filler powder. The amounts of the electro-conductive filler and the inorganic filler are in the respective ranges of from 20 to 100 parts by weight and from 25 to 300 parts by weight with respect to 100 parts by weight of the binder.
Japanese Patent Laid-Open Publication No. Hei 1-176319 (1989) discloses a back coat layer containing carbon black and an Al2O3 powder that has an a phase ratio of 40 to 70 wt % and has an average particle size of 0.3 to 0.6 μm.
Japanese Patent Laid-Open Publication No. Hei 10-11736 (1998) discloses a magnetic tape for recording computer data. The magnetic tape comprises of a 0.5 to 2.0 μm thick non-magnetic lower layer and a 0.05 to 0.5 μm thick upper layer, each disposed on one side of a non-magnetic support, and a back coat layer disposed on the other side of the support. The tape has a Young's modulus of 1200 kg/mm2 or more along the longitudinal direction of the tape. The back coat layer contains a soft inorganic powder that has an average particle size of 30 to 50 nm and has a Mohs hardness of 3 to 4.5, along with a hard inorganic powder that has an average particle size of 80 to 250 nm and has a Mohs hardness of 5 to 9. The total thickness of the magnetic tape is 7 μm or less.
As opposed to helical recording, current linear recording tapes for use with computers are run at a high tape speed of, for example, 2.5 m/s or higher and are therefore subjected to many problems during repeated runs. Such problems include the back coat layer being abraded by the guide roll, the resulting debris being taken up by the tape and transferred to the magnetic layer, and the magnetic layer being scratched and abraded during the high-speed sliding of the back coat layer against the magnetic layer. These problems cause an increase in the error rate and head-to-tape sticking and, thus, clogged head.
To solve these problems, magnetic recording medium comprising a back coat layer with even improved strength needs to be developed.